1. Field of the Invention
The present invention relates to fluidic systems, both closed and open systems, in which fluid temperature control at a point in the system is necessary or desirable. The present invention prevents fluid temperature from exceeding a predetermined set temperature at a point in the piping of a system, by cooling the flow in a controlled manner when required. One of the applications of the present invention is in the area of renewable energy, specifically solar thermal systems for water heating and space heating. Other applications include, but are not limited to, temperature clipping of: engine oil, engine coolant, transmission fluid, hydraulic fluid, cutting fluid, machining fluid, and fluid in a tank.
2. Description of the Prior Art
Many solar thermal heating systems suffer from overheating problems, including the loss-of-load problem, the over-supply problem, and the loss-of-flow problem. Loss-of-load and over-supply problems involve a mismatch in which the heat supply from the solar collector or collectors is greater than the heat load or demand. The loss-of-flow problem involves a loss or degradation of system fluid flow usually due to pump stoppage or slowing. Overheating of the fluid experiencing these problems sometimes leads to fluid breakdown, boiling and overpressurization in the solar collector(s), and consequently to damage to the solar collector(s) and other parts of the system.
Drainback and draindown type solar thermal systems deal with overheating problems using a control system to detect the overheat situation, then turn off the fluid pump allowing the fluid to drain out of the solar collectors. Other types of solar thermal systems deal with overheating problems in other ways. However, closed-loop solar thermal systems do not have a solid, proven means of dealing with overheating in all situations.
A classic loss-of-load solar-hot-water problem typically arises when a family goes on vacation in the summer without putting the solar system into vacation mode. With the water in a hot water storage tank already hot, and no calls for hot water, the system may overheat even with the system pump on because the solar collectors continue to add heat to the system which does not need it. Fluid breakdown, boiling and over-pressurization of the fluid in the solar collector usually follow, with accompanying damage to the solar collector(s), the solar thermal system and/or to the fluid itself in the case of antifreeze solutions. This is an example of the loss-of-load problem.
Some solar thermal system designers opt to degrade the solar-hot-water-heating system performance to provide almost all of the hot water in summer and about half of the hot water needed in winter to avoid overheating in summer. They choose to under-size the system to avoid overheating on the hottest summer days when the solar collectors may be producing at their highest heat levels. If designers were to design a system with more solar collector area for more hot water in winter, the system could produce an over-supply of heat at times in the summer, thus potentially leading to overheating and consequent fluid breakdown, boiling and over-pressurization of the fluid, and accompanying damage as described above. This is an example of the over-supply problem.
When the system fluid pump stops or slows down or for any flow degradation condition, the stagnant or nearly stagnant fluid in the solar collector on a sunny day may increase in temperature to the point where it breaks down and/or boils, again causing damage. This is an example of the loss-of-flow problem.
Others have attempted to solve these problems in different ways. US Patent Application Number 20100059047 describes an automated over-temperature protection system that uses a pressure vessel near the outlet of the solar collector. “ . . . in the event that fluid in the solar energy absorber vaporizes, the fluid is forced out of the solar energy absorber and into the pressure vessel.” This protection system fails to prevent boiling before it starts. The allowed boiling may damage the system or fluid in the system.
U.S. Pat. No. 7,823,582 describes an automated solar collector temperature controller which opens dampers to the air space of the flat plat solar collector. This protection system works only for flat plate solar collectors, and compromises the thermal integrity of the solar collector with openings and mechanical dampers which wear and eventually fail to close or open completely or properly.
U.S. Pat. No. 7,913,684 describes an automated protection system to remove vapor from a solar collector and indirectly cool it should fluid boiling occur in the solar collector in a pressurized solar thermal heating system. This system only addresses a loss of flow, only works for a pressurized system, and by itself fails to prevent boiling. The patent adds dampers to the flat plate collector in the same fashion as the patent above. This damper system works only for flat plate solar collectors, and compromises the thermal integrity of the solar collector with openings and mechanical dampers which wear and eventually fail to close or open completely or properly.
U.S. Pat. No. 4,102,325 describes an automated solar collector temperature control system which uses a thermosyphon, a valve and additional tubing integrated into and attached to the solar collector. This system is expensive and complex.
U.S. Pat. No. 8,459,248 describes a solar heating and cooling system that allows a thermosyphon loop to cool the fluid in the collector in pump-off situations. This system requires the system pump to be off to allow the cooling system to function. The system does not work for the loss-of-load problem, for the over-supply problem, for the partial-system-flow situation, nor any pump-on failure mode. “When the fluid pump is off, the working fluid circulates through the thermosyphon cooling loop, but when the fluid pump is on, the working fluid circulates through a heating loop.” Overheating may still occur with this system in pump-on failure modes. In addition, because the cooling assembly is “integral with” the back side of the solar collector, the system is not low-profile when flush-mounted to a roof.
Some solar thermal heating systems use separate heat dumps to shed excess heat. Typically, a heat dump may be a hot tub, a swimming pool, a slab of concrete with embedded hydronic tubing, a liquid-to-air heat dissipator, or other heat-dissipating device. Customary practice is to place a thermostatic valve downstream of the outlet of the solar collectors, and to divert some or all of the flow through the heat dump. This method overcools the fluid because there is no temperature feedback where the diverted flow returns to the system. This wastes energy, and results in longer times to bring the storage tank up to temperature.
Some solar thermal heating systems use multiple sensors, electrically operated valves, electronic control systems, and heat dumps to limit fluid temperature. These systems are generally complex, expensive and difficult to service and to diagnose when troubles arise.
Some solar thermal heating systems use periodic heat dumping by hot water discharge to bring the temperature of storage tanks back down to within operating range. These systems risk allowing tank fluid temperatures to get too high, and waste water by discharging hot water down the drain and injecting cold water. Such systems are potentially unsafe and wasteful of energy and water.
Solar thermal systems retrofit installations are infrequently done because of the expense and complexity of the installation. Much of the complexity and expense come from the lack of an available heat dump or the difficulty and expense of piping to a heat dump. When they are done, such retrofit installations typically are undersized to prevent overheating problems. Use of renewable energy in solar thermal systems is hampered by the complexity and expense of installation and is underutilized by undersizing.
Thermostatically controlled valves, both mixing valves and diverting valves, have been used for many years for fluid temperature control. For example, they are used for large diesel-engine-based electric generators to control coolant temperature to and from the engine and lubricating oil temperature to and from the engine. These valves typically combine coolant pumped from the engine with coolant from an external heat dump, usually outside the building housing the engine. These valve arrangements do not provide in-line cooling, and require an external source of cooling.
Thermostatic mixing valves are used in boiler-type heating systems for various purposes, including reducing the temperature of the fluid from the boiler going into a hydronic radiant floor. This system uses the return fluid from the radiant floor as the source of colder fluid. Such a cold return is not available in a solar thermal system and many other fluidic systems.
Thermostatic mixing valves are also used on domestic hot water systems to reduce the risk of scalding should the water heater produce water hot enough to burn the skin. This system uses the cold water source to reduce the water temperature. Such a cold source is not available in a solar thermal system.
Automobile engines typically use a thermostatic valve to allow coolant to leave the engine for cooling when the engine gets hot enough. However, typically the temperature of the coolant coming from the radiator and re-entering the engine is unregulated. This may produce cold sections in the engine and lead to increased wear.
Oil coolers for engines sometimes have a thermostatic valve where oil exits the engine. This valve sends oil back into the engine when the valve temperature is below a set value, but diverts the flow into a heat dissipator flow path when the valve temperature is above the set value, before returning to the engine. This system cools the oil when it becomes too hot, but does not control the amount of cooling as flow is simply directed into a heat dissipator without regard as to the fluid temperature exiting the heat dissipator. This results in overcooling the engine oil, especially in very cold climates.
Hydraulic systems need to operate within a small range of viscosity for proper operation and to avoid cutting component life short. This translates into maintaining the appropriate fluid temperature as viscosity is temperature dependent. Most current hydraulic systems simply have the operator watch for anomalous operation or watch temperature sensor gauges. When a high temperature issue arises and is detected, it is usually too late, with the result being that some hydraulic component malfunctions or fails. Current systems fail to prevent or mitigate high temperature conditions.
Cutting and machining fluids work best when they are at or below a predetermined temperature. Most current cooling systems for machines that use these fluids fail to limit temperature or maintain a constant fluid temperature.
In addition, current fluidic systems without electronic controls don't adjust to changing conditions such as ambient temperature, heat transfer rate from the heat dissipator, flow rate change from pump degradation, flow path blockage, or fluid temperature change. Adding electronic controls adds to the complexity of fluidic systems and adds extra expense.